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B Age of the universe: observable or entire universe?

  1. Aug 9, 2017 #1
    I was wondering if we are only aware of the observable universe, and the actual universe could be infinite, what do we mean when we say the age of the universe is 14 billion years? Are we talking about the age of only the observable universe or the whole universe?
     
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  3. Aug 9, 2017 #2

    phinds

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    The whole universe could be infinite, yes. That seems to be the preferred point of view these days but it is not an established fact at all. The age of the observable universe IS the age of the universe.
     
  4. Aug 9, 2017 #3

    phyzguy

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    By definition the part of the universe that is outside the observable universe is.....unobservable.
     
  5. Aug 9, 2017 #4
    The observable universe is what is 14bn or so years old.
    That does not mean it is all there is.
     
  6. Aug 10, 2017 #5
    Thank you for clarifying!
     
  7. Aug 10, 2017 #6

    kimbyd

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    The way that the age is usually defined is by taking the classical Big Bang model and extrapolating back in time. Go far enough back in time, and the model says there is a singularity. The age is defined from that point.

    However, the singularity should not be understood as being a real thing. It's the point where something happens which we don't quite understand. One popular proposal for dealing with the singularity is cosmic inflation. Cosmic inflation pushes back the time scale of the universe by a tiny fraction of a second before the Big Bang singularity. But its nature is such that it hides whatever happened before.

    So the best way to understand it, to me, is that an event happened roughly 14 billion years ago which hides the nature of whatever happened before that point. To put it another way, anything that may or may not have happened before roughly 14 billion years ago is outside our observable universe. It's possible some very interesting things happened prior to inflation, or that inflation was the actual start of our universe. We just don't know.
     
  8. Aug 10, 2017 #7
    Thank you! I have copied this answer into my notebook :)
     
  9. Aug 10, 2017 #8
    after?
     
  10. Aug 10, 2017 #9

    kimbyd

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    Before.

    The Big Bang singularity comes from a model without inflation. Inflation changes the early model, removing that singularity. It's possible to prove that inflation had to start at some point (it can't be eternal into the past), but the precise timing of that event is hidden.
     
  11. Aug 10, 2017 #10

    PeterDonis

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    Can you be more specific about what you're referring to here? Are you saying that an inflating spacetime has to have a past singularity? Or just that an inflating spacetime has to start from some non-inflating background?
     
  12. Aug 10, 2017 #11

    George Jones

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    What about the Borde-Guth-Vilenkin "singularity" theorem,
    https://arxiv.org/abs/gr-qc/0110012

    Abstract:
    As far as I know, there is no reasonably generic, accepted definition of "spacetime singularity". There is, however, a reasonably generic definition of "singular spacetime". A rough, sufficient condition: spacetime is singular if there is a timelike or null curve having bounded acceleration that ends in the past or the future after a finite amount of proper time or affine parameter.
     
  13. Aug 11, 2017 #12
    I think it is simply unjustified to say the universe is 14 bio years old. I think cosmologists are seriously misleading the public with this statement.
    The correct statement should be we can trace the universes evolution back 14 bio years, before that we dont understand the physics. and we are unlikely to do so unless we have a well checked (by experiment) quantum theory of gravity . That is something we are lacking at the moment So the universe is at least 14 bio year old but might be much older or could even be infinitely old, we don't know.
     
  14. Aug 11, 2017 #13

    kimbyd

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    The model of inflation unambiguously predicts a past singularity (this is distinct from the Big Bang singularity, but related).

    One way to understand this past singularity is that as inflation progresses, the universe becomes exponentially more dilute. This is why inflation explains the horizon problem: it makes a region much larger than the observable universe almost perfectly uniform by its nature. If you instead run time the other way, and ask what inflation looks like into the past, then the answer you get is if there are any contents in the universe at all during inflation, even a single photon, will result in a past singularity. Even just a slight uneveness in the inflaton field itself will cause this. Thus you are saddled with two possibilities:
    1. Inflation is perfectly finely-tuned, being perfectly uniform and with no other matter (including photons) in the universe.
    2. There is a past singularity in the model (at an unknown time).

    Generally the first possibility is considered absurd enough to discount entirely, leaving the past singularity. That past singularity has the same general class of possibilities as the Big Bang singularity. One possibility is that there are some unknown physics that resolve the singularity. Another is that there was an event that occurred that isn't captured by simply extrapolating inflation back in time.

    This line of argument is why some physicists claim that inflation doesn't solve certain problems it claims to solve. The hope of people advocating for inflation models is that the uncertainties about how inflation began are somehow easier to explain than the Big Bang singularity. This isn't proven, but it's reasonable on the surface: the event that started inflation would have had to occur over a much smaller region of space, lending hope that it's easier to come up with a physical process that will do that. At this point, though, exactly what would explain Inflation's past singularity is speculation.
     
  15. Aug 11, 2017 #14

    PeterDonis

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    Ah, I see. So technically there is a difference compared to non-inflating FRW spacetimes (which have a past singularity even if everything is exactly homogeneous and isotropic), but it's not going to make any difference in a practical sense.
     
  16. Aug 11, 2017 #15
    I Think there is more controversy here than you let on. Borde Guth Vilenkin claimed to show that inflation can't be past eternal but this has been disputed by others such as Aguirre, Nomura, Susskind etc . Since there is no way to experimentally confirm which is correct I think this is much more of an open question.
     
  17. Aug 11, 2017 #16
    Inflation does make some sense to me.
    Yet it also seems just as kicking the can down the road.
    There still is a singularity, which means something is going on which we don't understand.
     
  18. Aug 11, 2017 #17

    kimbyd

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    It is in a sense. The idea is that the beginning of inflation is somehow easier to explain than the start of the classical Big Bang model. One argument along these lines is that inflation basically guarantees a smooth, spatially-flat universe. It doesn't solve the singularity problem (as it has its own, related singularity), but the hope is that singularity is easier to explain.

    Currently, the most significant evidence for inflation comes from what is known as the nearly scale-invariant power spectrum.

    Inflation gets is explanatory power from the fact that during inflation, the energy density of the universe was nearly constant over time. Nearly, but not quite: in order for inflation to end, the energy density had to change at least somewhat over time. If the density had been exactly constant right up until the end of inflation, then we would have what's known as a "scale-invariant power spectrum". This comes from the fact that large-scale fluctuations came from earlier periods, smaller-scale fluctuations came from later periods. In a model where the energy doesn't change at all, those fluctuations all have the same average amplitude. But when the energy changes over time, that amplitude changes. Inflation predicts that the energy will have changed slowly over time, leading to a small (but measurable!) difference between the amplitudes of the large-scale and small-scale fluctuations. This has been measured.

    However, this evidence is somewhat limited. There are a number of other precise details of the fluctuations that stem from inflation that can in principle be measured, but many of them vary so much between inflation models that it's often hard to say whether or not inflation as a whole is correct. However, our current data is detailed enough that a number of the proposed inflation models don't seem to fit. My hope is that over the coming decades, we'll get more precise data and will be able to say whether any of these inflation models (or a different model altogether) are correct.
     
  19. Aug 12, 2017 #18

    kimbyd

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    This gets a little bit into some rather nitty gritty details. Susskind*, for example, is arguing from the perspective of string theory with multiple vacua (source: https://arxiv.org/abs/1205.0589). This goes quite a bit beyond the simple inflation models discussed so far. I would argue that the past-finite conclusion above is not seriously impacted by arguments such as these. Expanding this discussion to include arguments like Susskind's is really complicated and I'm not sure it really should be discussed in detail in this thread.

    My general view is that the specific instance of inflation that started our observable universe (if it was inflation) is almost certainly past-finite. That instance may be connected to another space-time in the past, and that space-time may have also been an inflating space-time. Described from this perspective, Susskind's model would have an infinite series of inflating regions each with different vacuum energy. Each individual inflating region would be finite. But the whole structure would be effectively past-eternal.

    Some argue that even models like Susskind's must also be past-finite. That's a much more debatable argument, as you mention.

    *I focused just on Susskind's argument in this post to keep the discussion a bit compact.
     
  20. Aug 16, 2017 at 7:57 AM #19
    How is the age measured? According tot GR time clicks different for clocks in different regions. Which clock reads 14 billion years?
     
  21. Aug 16, 2017 at 8:01 AM #20
    That is not true. Every clock ticks 1 second per second.
     
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